The proper positioning of the cell division plane is critical for cellular proliferation, and influences developmental processes such as cell differentiation, establishment of tissue architecture, and cell morphogenesis. The basic understanding of mechanisms of cytokinesis has broad relevance to human disease in areas such as cancer biology and stem cell biology. The general goal of this grant is to determine basic, fundamental mechanisms of cytokinesis and cell polarization. For our studies, we use a simple, genetically tractable model organism, the fission yeast Schizosaccharomyces pombe. A key process in division site placement is the localization of a cytokinesis factor mid1p to a medial cortical region overlying the nucleus. During mitosis, mid1p recruits other cytokinesis factors to this site to initiate the assembly of the actin-based contractile ring. We have discovered that diverse cellular components contribute to the localization of mid1p to this cortical location. Our specific aims focus on characterizing new components that regulate this process: the endoplasmic reticulum, and a gradient of the pom1p kinase, which is indirectly set up by microtubules. Our innovative approaches include measuring the dynamic behavior of proteins inside of living cells and altering cell shapes using micro-fabricated chambers. These studies promise to provide a significant advance in the quantitative understanding of this conserved, universal process and elucidate general concepts of global spatial regulation used in building the cell.